A TEMPO-catalyzed oxidation-reduction method to probe surface and anhydrous crystalline-core domains of cellulose microfibril bundles

Authors: SHIGA, TANIA - Universidad De Sao Paulo; YANG, HAIBING - Purdue University; Penning, Bryan; OLEK, ANNA - Purdue University; MCCANN, MAUREEN - Purdue University; CARPITA, NICOHLAS - Purdue University

Submitted to: Cellulose
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2021
Publication Date: 4/15/2021
Citation: Shiga, T.M., Yang, H., Penning, B., Olek, A., McCann, M.C., Carpita, N.C. 2021. A TEMPO-catalyzed oxidation-reduction method to probe surface and anhydrous crystalline-core domains of cellulose microfibril bundles. Cellulose. 28:5305-5319. https://doi.org/10.1007/s10570-021-03815-9.
DOI: https://doi.org/10.1007/s10570-021-03815-9

Interpretive Summary: In order to predict the ease of digestion of cellulose, its structure within plant cell walls must be better understood. Different measurement techniques have indicated different numbers of strands of cellulose being bundled together with different exposure to water of the individual glucose sugars that make up each cellulose strand. More crystalline cellulose structures have lower exposure to water molecules and are harder to digest. This research combines several existing chemical techniques to better understand the size of bundles of cellulose and water exposure of individual sugars. Findings indicate that cellulose bundles in cell walls are larger than expected, up to 200 strands of cellulose in a bundle, reducing their digestibility. Depending on the part and type of plant, 67-86% of cellulose can be in crystalline form with cotton cell walls the highest and Arabidopsis leaf and poplar wood cell walls lower. Other molecules that interact with cellulose bundles such as lignin in poplar tree wood reduce the crystallinity to ~72% but interfere with digestibility themselves. However, altering the type of lignin in poplar tree wood can further reduce crystallinity to ~67% leading to increased digestibility. This research shows that the structure of cellulose itself is partially responsible for the difficulty in digesting cellulose from plant cell walls. Knowledge of the structure of cellulose will benefit scientists and engineers employing strategies in digestion of cell walls for conversion to bioproducts or fuels. Researchers will benefit from the knowledge that altering lignin content or structure in cell walls also affects the structure of cellulose bundles and thus how easily they digest both commercially and potentially in grass-eating animals.

Technical Abstract: A modified TEMPO-catalyzed oxidation of the solvent-exposed glucosyl units of cellulose to uronic acids, followed by carboxyl reduction with NaBD 4 to 6-deutero and 6,6-dideuteroglucosyl units, provided a robust method for determining relative proportions of disordered amorphous, ordered surface chains, and anhydrous corecrystalline residues of cellulose microfibrils. Both glucosyl residues of cellobiose units, digested from amorphous chains of cellulose with a combination of cellulase and cellobiohydrolase, were deuterated, whereas those from anhydrous chains were undeuterated. By contrast, solvent-exposed and anhydrous residues alternate in surface chains, so only one of the two residues of cellobiosyl units was labeled. Although current estimates indicate that each cellulose microfibril comprises only 18 to 24 (1 , 4)- ß -D-glucan chains, we show here that microfibrils of walls of Arabidopsis leaves and maize coleoptiles, and those of secondary wall cellulose of cotton fibers and poplar wood, bundle into much larger macrofibrils, with 67 to 86% of the glucan chains in the anhydrous domain. These results indicate extensive bundling of microfibrils into macrofibrils occurs during both primary and secondary wall formation. Beyond lignin, the degree of bundling into macrofibrils contributes an additional recalcitrance factor to lignocellulosic biomass for enzymatic or chemical catalytic conversion to biofuel substrates.